By far the most expensive opossum repellents on the market are ultrasonic. The price tag does come with something of a guarantee, though, as this method generally works best. They work by emitting a high-frequency sound that is inaudible to humans, so you don’t have to worry about being affected by it, personally.

Many animals are affected by high pitched sounds, and these devices are usually marketed as a way to prevent many types of animals from making your yard their home. That can also be the downside of these opossum repellents–maybe you only want to repel opossums but keep the birds and squirrels. These frequencies can affect pets, as well, so pay attention to your dogs to see if they are reacting negatively to it.

There are two ways these ultrasonic opossum repellents work: constantly or motion-triggered. Again, it’s outside the range of human hearing so you most likely won’t recognize when the sound is on. If it’s on constantly, then you won’t have to worry about whether it’s taking effect or not because you will be able to see the difference. If it’s motion-triggered, you’ll allow other wildlife into your yard, but its efficacy might be in question depending on when, where, and how it’s triggered.